Method and apparatus fob the



Feb. 16, 1937. 1L D NBAR Er AL Re. 20,266

METHOD AND APPARATUS FOR THE DIGESTION 0F FIBROUS MATERIAL Original Filed Nov. 20, 1951' Reissued Feb. is, 1931 METHOD AND APPARATUS FOR THE DIGESTION OF FIBROUS MATERIAL Thomas Leonidas Dunbar and Albert Darius Merrill, Watertown, N. Y.

Original No. 1.918.459, dated July 18,1933, Serial No. 576,255, November 20, 1931. Application for reissue January 29, 1935, Serial No. 3,984

v 39 Claims.

The present invention relates to an improved method and apparatus for the digestion of fibrous material, such as wood chips.

In the digestion of fibrousmaterial, it has been proposed to recover the gases and liquors exhausted from the digesters during the cooking process and to utilize the same to strengthen and heat the raw acid liquor before it is fed into the dlgesters for the next cook.

One of the objects of our invention is to provide a method of-and apparatus for further heating the acid liquor before its entrance into the digesters, so that it is brought to the most edvantageous temperature without substantial loss S02.

Another object of our invention is to provide means whereby the pressure in the accumulator is maintained substantially constant.

It is a further object of our invention to pro- 20 vide improvements in the construction and operation of the apparatus employed in the digestion of fibrous material whereby uniformity in the results obtained may be materiallypromoted, economies in construction and operation effected 35 and the efilciency of operation increased.

Having in view these objects as well as others which will appear as the description proceeds, the

invention consists in the novel features hereinafter described in detail, illustrated in the accompanying drawing and .more particularly pointed out in the appended claims. In the drawing: Fig. 1 shows in elevation one apparatus form- I 45 nections shown in Figs. 2 and 3, so that if normally operated the tower that is the weak acid tower one day will be the strong acid tower the next day. Figs. l and 2 showthe tower I as the weak acid tower. In this case (referring espe- 50 cially to Fig. 2), the 6-way valves 26 and 26 are set so that water coming through the line 3, which is the suction line from the fresh water supply. passes from the valve 26 through the 4 pump PI the discharge line 4 of the pump PI the 55 6-way valve 26 and the line 34 to the top of the ing the subject. matter of the present invention acid tower I. After passing through the tower I,

' the settling tank 6.

When 2 is operated as the weak acid tower and 10 I as the strong tower, the valves 25 and 26 are set as shown in Fig. 3. In this case the water supply passes through the line 3, valve 25, pump P2, line 6, valve 26 and line 35 to the top of the tower 2. From the bottom of this tower the liquor passes through the line I, valve 25, pump PI, line 4 and valve 26 to the exchange heater A, and thence through the valve 26 and line 34 to the top of the tower I. From the bottom of this tower it passes through line 5, valve 25 and line 36 to the settling tank 8, all as shown in Fig. 3.

The pump P3 takes the acid liquor from the settling tank 6 and discharges the same through the line 9 into the acid storage tank Ill. From the tank III the acid liquor is withdrawn by the pump P4 and forced through the line II, the exchange heater B and the check valve Ila. to the eductor EI. 'This eductor is provided with a regulating valve and is of the type shown and described in our copending application Ser. No. 528,696, flied April 8, 1931. I2 is a drop or condenser leg which carries the mixed fresh acid liquor and hot digester relief fluids from the eductor EI to the eductor E2. The eductor E2 is similar to eductor El and may or may not be provided with a regulating valve.

The pump P5 is provided with valved suction lines I! and I6 so that acid can be pumped either from the storage tank III or from the accumulator 40 i4 through the discharge line I1 and the eductor E2. From the eductor E2 the liquor passes through the exchange heater C and the line I! into the accumulator I4 near the bottom thereof. The accumulator I4 is spherical in shape and is provided with a gauge glass 32 or other liquid level indicator.

The suction line I 6 leads from the bottom of the accumulator to the pump P6, which is used for filling the digesters by forcing the hot preconditioned acid from the accumulator through the exchange heater D and thence through the discharge line I9 and the valved branches 2!! to the digesters 2|. The thermostatic control 36 has its element located in the line I9 between the heater D and the branches 20, and is connected to a valve in the heat supply (for example steam) line 39 to the heater D, so as to control the heat supplied at this point. Thus when the temperature of the acid in the line l9 exceeds a certain point, say 110 (2., the thermostatic control will operate to shut oif further heat supply to the heater D, and when this temperature drops below another point, say 105 C., the steam or other heating medium will be automatically admitted again. Similar thermostatic controls, set to operate at different temperatures to be discussed later, may be connected to the other exchange heaters A, B and C, if desired. Since such controls are well known in the art, it is not necessary to describe them in further detail. nor to indicate their presence at more than one point in the apparatus.

Each digester is provided with a gauge for accurate determination of the liquid level therein,-

either a gauge glass as shown at 31,.or an indicating or recording liquid level gauge.

The valved relief branch 29 leads from the top of the digester to the header 30, which header discharges into the acid storage tank Ill at a point approximately four feet below the normal liquid level in this tank. From the branch 29;

, the valved relief branch 3| vents immediately to the atmosphere.

The valved relief branch 22 leads from the top of the digester to the main relief header 24 leading to the eductor El. The valved relief branch 23 leaves the digester at a point a little below the normal liquid level maintained in the digester, and exhausts into the header 24. A bypass line 28 leads from the header 24 to the top of the accumulator l4. The pressure regulating valve 21 in the line 28, the operation of which will be more fully described at a later point, automatically vents the relief gases and liquor from the header 24 directly into the top of the accumulator when the pressure in the accumulator drops below a predetermined point.

The pressure regulating valve 4| in the line 42 vents gases from the top of the accumulator into the contents of-the acid storagetank whenever the pressure in the accumulator rises above a predetermined point (normally about 30 pounds). The line 42 leads from the top of the accumulator over and down into the contents of the acid storage tank to the depth necessary for absorption of the vented gases.

In the practice of our improved method, the digester is filled with chips from a bin or conveyor (not shown) by removing the manhole cover at the top of the digester. After the dlsester has been filled with chips, the manhole cover is bolted on and all connections made to the relief lines.

The acid liquor which is next pumpedinto the digester has been preheated not only by direct absorption of heat from the relief gases and liquors from the digesters. but also by the exchange heaters A, B, C and D. By reason of the higher temperature, thus obtained. of the acid enterin the digester, less steam is required to bring the contents of the digester to the desired temperature, resulting in a saving of steam and a reduction of the dilution of the acid by steam con-v densation. While it has been proposed to heat the acid liquor by means of exchange heaters before its entry into the digester, prior methods of accomplishing this either fail to raise the acid to the most desirable temperature at the point of its entry to the digester, or result in loss of from the acid, or both. Our new method and apparatus present the very important advantage over methods pra'cticed up to the present time, that we so heat the liquor throughout the system that we maintain its $0: content while raising it to the most advantageous temperature by the time'it enters the digesters. The exchange heaters shown in the accompanying drawing are of a type known to" the art, and heat may be supplied thereto in any convenient manner, such as by utilizing hot relief liquors from the digesters. by steam, or in any other suitable man-' ner. The phrase "indirectly heating" in the appended claims is employed to designate heating, as by an exchange heater, in which the heating and the heated mediums do not come into direct contact with each other, as distinguished from direct heating such as takes place when the hot relief gases and fluids from the digester system come into direct contact with the acid liquor to be heated in the drop leg l2.

The heater A is located between the weak and the strong towers of the acid system, and by itsv use we are able to control the combined SO: in the system acid. The higher the temperature in the acid making system, the higher will be the combined 80: in the acid. If, however, the

temperature in the acid making system should run too high or if the acid going to the weak towerwere heated, considerable 80: would be lost, as the weak acid tower is open to atmospheric pressure at the top to vent CO: gases formed by the reaction between the limestone, water and SO:

. gas. By placing the exchange heater A between the weak and strong acid towers, we can control the temperature and thereby the combined and total 80: percentages in the system acid, thereby accomplishing a result which has long been desired in maintaining the strength of the system acid more nearly constant. By maintaining the system acid at the desired percentages of free and combined S02, we make for more uniform resiilts in the process since the uniformity of the cooking acid is to a large extent dependent upon.

the uniformity of the system acid.

The acid in the storage tank l0 being at atmospheric pressure-this acid should not be at a temperature higher than 45 C.. as otherwise it would not hold its free SO: when agitated by the flow of relief fluids entering the body of liquor. At more elevated temperatures, there will be a loss of 80: unless the pressure on the acid is correspondingly increased above atmospheric. Further, since the tank In is' ordinarily constructed of wood, a mild cooking of the wood tank would result if the temperature of the acid were in excess of this point,rthereby shortening the life of the tank. v

The acid in the line I I is under pressure. as the pump P4 delivers its rated capacity against a total head of approximately 300 feet (or pounds pressure).

The acid in this line can therefore be further heated by the heater B without loss of fore located at this point to accomplish this result.

The pressure carried in the line I3 is normally higher than the pressure carried in the accumulator l4. .Heat can therefore be again applied to the acid as it is being pumped to the digester, by the heater D. The contents of the digester should not be raised to a temperature above 110 C. before the acid has penetrated into the raw material as surface delignification would take place, thereby weakening the acid and preventing the penetration of acid of a high concentration into the center of the chips. .This results in too high a percentage of uncooked wood when the finished pulp is produced. The thermostatic control 38 is therefore provided which may be set so as to shut on the heat when the temperature of the acid in the line l3 reaches 110 C., the

heat coming'on again when the temperature drops to 105 C.

In filling the digesters with acid, acid is withdrawn from the accumulator by means of the pump P6, and is forced through the exchange heater D, the line l9 and whichever one of the valved branch lines 20 leads to the digester which is being filled. In the usual operation of sulphite mills, only one digester is filled at a time, it having been found in general best to have approximately an equal time interval between the several digester filling operations. While the digester is being filled with acid, the valves in the branches 22 and 23 are closed and the valve in the I as the liquor rises therein. As soon as gas (which is readily detected by its odor) appears at the vent of the branch 3l, the valve in the branch 3| is opened and the valve in the branch 23 is opened. This passes the gases that rise from the digester during the filling operation, by means of the branch 23 and'the header 30, back to the acid storage tank l where they are recovered, thereby avoiding smell in the digester room and also saving the gases. Thus the term fresh acid in this specification and in the appended claims may include acid which has been partially preconditioned by absorption in the storage tank of these gases.

As soon as the digester has been solidly filled with liquor, the valve in the branch 23 is also closed and a pressure of about 50 pounds is placed on the digester by means of the pump P8. As soon as this pressure has been reached, the valve in the branch 20 is closed and all other valves of the digester are also closed for a soaking period of approximately one hour.

, At the end of this soaking period, steam is admitted to the bottom of the digester through a steam line (not shown) and the valve in the relief line 22 at the top of the digester is opened, thereby venting fiuids to the main header 24 and thence to the eductor El. After a period of time varying with the wood conditions-particularly the moisture content of the woodthe valve in the line 22 is closed and the valve in the line 23 is opened in order to draw of! relief liquor from the side of the digester.

is left open until the liquid level in the digester has been lowered to the proper point. The appearance of dry gas at the top of the digester indicates that the level is approaching this point. The presence of dry gas at the top of the digester .content of the steam.

the same installation.

The relief gases and liquids entering the head- The valve in the branch 23 is readily determined by slightly opening the valve in the line 22, as the sound produced by dry gas passing through this line is considerably different from that produced by wet gas or liquid.

The steaming of,the digester dilutes the acid somewhat and after the valve in the branch 23 is closed raises the liquor level in the digester, due to steam condensation. The extent of this condensation varies with the moisture In any event, it is important for uniform results that the liquor level in the digester at the end of the cook be very nearly constant in all cases. vide each digester with a gauge glass or other indicating liquid level gauge and/or with a recordingliquid level gauge of a type known in the art. The indicating liquid level gauge is convenient for the mill operators, since it can be read from a considerable distance, whereas the recording gauge gives a record enabling the superintendent or the technical department to. check afterwards on the proper operation of the digesters. By the use of a level gauge and by proper manipulation of the valves in the lines 22 and 23, the final liquor level in the digester may be substantially standardized at a definite point, say five or six feet below the top manhole cover plate. It will be evident that the higher the moisture content of the steam used in the steaming of the digester, the more liquor will be withdrawn through the side relief branch 23, in order that the final liquor. level may be at the standardized point. This final level may vary in different installations and with differing operating conditions, so long as it is substantially standardized for the same set of conditions in er 24 through the lines 22 and 23 are conducted by the header 24 to the eductor El, where they meet the flowing streamof liquor coming through the line H to this eductor, and contacting and passing therewith down the drop leg l2 to the eductor E2 they are absorbed by and give up heat to this liquor. The two eductors El and E2 permit us-to carry a much higher pressure in the 'accumulatorthan is carried in the relief header 24, and our improved apparatus therefore requires only one main relief header. v

The accumulator I4 is spherical in shape. Thus smaller surface area is required for a given capacity, thinner steel plates will sufiice for a given working pressure, and the cost of the accumulator installation is materially reduced. An additional and very important advantage of our spherical accumulator is that we obtain a better dropleg effect since the height of the liquid level for a given capacity is greatly reduced.

We normally operate the accumulator so that the acid level in same has reached a point from six to ten feet below the top of the sphere at the the operation known to the prior art, the pres-' We therefore prosure in the accumulator drops from 10 to 25 pounds during the time a digester is being pumped. As the pressure drops, the acid in the accumulator gives up an amount of its free SO:

gas directly proportional to the drop in pres- .relief header 24 exceeds by one pound diii'erential that in the line 2., the valve 21 opens and gases are by-passed around theeductor El and directly into the acid accumulator. Thus when a digester is being filled and there is a tendency for the pressure in the accumulator to drop, the valve 21 opens and permits the relief fluids from the other digesters to vent directly into the accumulator through the by-pass II, maintaining the pressure in the accumulator.

when the pressure inthe accumulator rises above a predetermined point (normally about 30 pounds), the valve. in the vent line 42 relieves gases from the top of the accumulator into the acid storage tank, preventing excessively high, pressure in the accumulator. At the same time, conducting these accumulator gases into the storage tank prevents the escape into the atmosphere of these gases, which are obnoxious and destructiveto human and vegetable life. Further, by conducting these gases into units thereof.

1, In the digestion of fibrous material by the sulphite process. progressively raising the temperature of acid liquor as the pressurethereon progressively increases during its from the weak acid tower to the digester and utilizing the heated liquor for the digestion of fibrous material. m I g a 2.. In the digestion of fibrous material by the sulphite process, intermittently and indirectly heating the acid liquor as the pressure thereon is intermittently and progressively increased during the course of its passage from the acid makin system to the digester system;

3. In the digestion of fibrous material by the sulphite process, progressively raising the acid liquor to a temperature above 100 C. during its passage from the acid-making system to the di gester without substantial loss of free 80: by increasing the temperature of the acid in proportion to the pressure to which it is progressively subjected during the course of such passage to the digester.

4. In the preparation of acid liquor for the digestion of fibrous material by the sulphite process, controlling the so, percentages inthe system acid by regulated heating of' the acid after amass it has passed through the weak acid tower and before it enters the strong acid tower.

5. In the preparation of acid liquor for the digestion of fibrous material, indirectly heating the acid during its passage through the raw acid system.

6. In the preparation of acid liquor for the digestion of fibrous material, heating the system acid after it leaves the weak acid tower and before it enters the strong acid tower.

7. In the preparation of acid liquor for the digestion of fibrous material by the sulphite process, heating the acid during its passage through the raw acid system to such a degree as to obtain necessary combined SO: content without substantial loss of free S02. e

8. In a process of the character described, in-

creasihg the acid strength of fresh liquor by conpors discharged from the digester in the liquor in the accumulator.

10. 'In a process of the character described, forcing acid liquor under pressure from an accumulator into a digester, indirectly heating the liquor while under such pressure and before it enters the digester, and automatically controlling the temperature of the liquor entering the digester within fixed limits.

11. In a process of the character described. contacting acid liquor in a conduit with relief gases and liquids from a digester system, passing the acid liquor after such contact into a pressure accumulator, withdrawing liquor from said accumulator and when the withdrawal of acid liquor from the accumulator lowers the pressure therein below that in the relief line by a predetermined differential, passing relief gases and liquids from the digester system directly into the accumulator without previous contact with the acid liquor.

12. Ina process of the character described, contacting 'acid liquor ina conduit with relief gases and liquids from a digester system, passing the acid liquor after such contact into a pressure accumulator and substantially maintaining the pressure in the accumulator by admitting gases tently passing relief gases and liquids from the digester system directly into the accumulator to maintain the pressure therein 14. In the digestion of fibrous material, charging a digester with hot acid liquor and fibrous material, and releasing acid-liquor from the side;

- of the digester untilsufiicient liquor has been relievedso that at the end of the cook theliquor some end of the cook.

16. In an apparatus of the character described, a weak acid tower, a strong acid tower, means for passing acid liquor from the weak to the strong tower, and means for heating the acid liquor after it leaves the weak tower and beforev it enters the strong tower.

17; In an apparatus of the character described, two acid towers reversibly connected in series scthat acid-liquor may be passed from one tower to the other and an exchange heater so connected as to heat the acid liquor in the course of its passage from one tower to the other.

18. In an apparatus of the character described. an acid storage tank, a conduit connecting said tank to a drop leg having an eductor at either end thereof, a pump for forcing acid liquor under pressure from the storage tank to the 'headoi the drop leg, and means interposed in said conduit between the pump and the eductor at the head of the drop leg for heating the liquor.

19. In an apparatus of the character described. a drop leg having an eductor at either end thereoi, a conduit leading from the eductor at the bottom of the drop leg to a pressure accumulator, and means for heating acid liquor in the course of its passage along said conduit from the bottom eductor to the accumulator.

20. In an apparatus of the character described, a pressure accumulator, a digester, a conduit for acid liquor leading from said accumulator to said digester, means for heating the acid liquor after it leaves the accumulator and before it enters the digester and a thermostatic control for automatically regulating the amount of heat imparted to the liquor by the said heating means.

21. In an apparatus of the character described, a weak acid tower, a strong acid tower, an acid storage tank, a drop leg, an accumulator and a digester, all connected in series, and means for heating acid liquor after it leaves the weak acid tower and before itenters the strong acid tower, after it leaves the storage tank and before it enters thedrop leg, after it leaves the drop leg and before it enters the accumulator, and after it leaves the accumulator and before it enters the digester.

22. In an apparatus of the character described, a pressure accumulator, a conduit for feeding acid liquor to the accumulator, a jet eductor interposed in said conduit, a digester system vent header connected to said jet eductor, a by-pass conduit leading from the digester system vent header to the accumulator and an automatic pressure regulating valve in the by-pass conduit.

23. In an apparatus of the character described,

- a pressure accumulator, a passageway for feedduit when the pressure in the digester system vent header exceeds by a fixed differential the pressure in the accumulator.

24. In an apparatus of the character described,

a spherical accumulator, a conduit having interposed therein a drop leg with Jet eductors on both ends thereof for feeding acid liquor to said accumulator and a digester system vent header connected to the jet eductor at the head of the drop leg.

25. In an apparatus of the character described, a pluralty of digesters each provided with a liquid level gauge, a spherical accumulator, a conduit having interposed therein a drop leg with jet eductors on both ends thereof for feeding acid liquor to the, accumulator and a single relief header connecting the digesters with the accumulator through the eductor at the head of the drop leg.

26. In a process of the character described, contacting acid liquor with relief gases and liquids from a digester system, passing'the acid liquor after such contact into a pressure accumulator, and minimizing pressure variations in the accumulator by admitting gases and liquids from the digester system thereto directly and without previous contact with the acid liquor when the pressure in the accumulator drops below the pressure in the digester system relief line by a predetermined diflerential, and by relieving gases from the accumulator when the pressure therein exceeds a predetermined point.

27. In a process of the character described, passing acid liquor under pressure from a storage tank to means for absorbing gas under pressure, and raising the temperature of the liquor approximately C. in the course of its passage from the storage tank to the said gas absorption means.

28. In the digestion of fibrous material, digesting successive charges of fibrous material under heat and pressure in a digester, and terminating the cooking of successive charges of fibrous material of substantially the same characteristics with the .liquor in the digester at substantially the same level.

29. In apparatus of the character described, a digester provided with a liquid level gauge operative to indicate the liquid level within the digester during the cooking operation, means for admitting cellulosic material to and withdrawing pulp from the digester, means for filling the di-' gester with liquor and means for withdrawing liquor from the digester to control the height of the liquor level therein.

30. In a process of the character described, digesting cellulosic material in a digester with a digesting liquor containing a volatile component;

establishing a volume of such liquor in a separate receptacle, recirculating the liquor to and from such receptacle, heating suchrecycle liquor exteriorly of the receptacle, and utilizing the recycle stream to withdraw the volatile component from the digestion zone and admix with the recycle liquor stream. 31. In a process of the character described, di

gesting cellulosic material in a digester with a digesting liquor having a volatile component;

establishing a volume of the liquor in a separate pressure receptacle, recirculating the liquor directly to and from the receptacle, heating the recycle stream exteriorly of the receptacle with hot reaction products, and utilizing such recycle stream. to withdraw volatile components from the digestion zone and admlx them with the recycle liquor stream.

32. In a process of the character described, digesting cellulosic material in 'a' digester with a digesting liquor having a volatile component; es-

tablishing a volume of the liquor in a separate pressure receptacle, recirculating the liquor direceptacle, directly heating said recycle stream by 6 rectly to and from the receptacle. indirectly heating the recycle stream exteriorly of thereceptacie with hot reaction products, and utilizing such recycle stream to withdraw volatile components -from the digestion zone and admix them with the recycle liquor stream;

33. In a process oLthe character described. establishing a volume of digestion liquor in a pressure receptacle, withdrawing a stream of liquor from the receptacle and forcing it back into the admixing therewith hot reaction products from'a digestion operation, and after such admixture gesting cellulosic material in a dlgester with a digesting liquor which includes a volatile component; establishing a volume of such liquor in a separate receptacle, recirculating the liquor to and from the receptacle and indirectly heating the recycle streamlof liquor exteriorly of the reoeptacle. utilizing such stream to withdraw volatile components from the digestion zone and ad- .mix such components with the recycle liquor stream, and introducing relatively cool liquor an acid liquor accumulator oi suiflcient capacityto carry several digester charges, a digester,

means for forcing the liquor under pressure from the receptacle to the digester, and means for heating the liquor while under such pressure and before it enters'the digester.

38. In an apparatus of the class described. a digester provided with means to indicate the level of liquid therein throughout the cooking operation, means to charge celluloslc material and digestion liquor to the digester, and a liquid relief line operative to withdraw liquor from the digester during the cooking operation to a predetermined level'as indicated by said means.

39. In the digestion of fibrous material, admitting a charge of cellulosic material and digestion liquor to a pressure digester, establishing cooking conditions of temperature and pressure within the digester, associating with the digester a device adapted to indicate the level of the liquid therein throughout the cooking operation; relieving liquor from the digester during the cooking to a predetermined level as indicated by said liquid level device.

THOMAS LEONIDAS DUNBAR. ALBERT DARIUB mm 

